The youngster's change of heart has come as a surprise to Football Australia (FA) staff, who had been monitoring his performances for English Championship side Portsmouth in the build-up to March window fixtures against Cameroon and Curacao.
FA was always aware of the risk that Segecic, who has played at every junior international level for Australia, would potentially switch to Croatia.
But FA only got a chance to speak to the attacking midfielder after he had committed to Croatia.
Sydney-born Segecic was handed his maiden Australian call-up by Tony Popovic last September, but did not see game time.
Segecic was not included in October's friendlies against the US and Canada, and was ruled out of the November international window with an ankle injury.
Under FIFA rules, once Segecic is fielded in a match by Croatia, he is unable to switch back to Australia.
If he didn't play for Croatia and wanted to backflip on his decision, he would need to submit a request for change of association to FIFA's Football Tribunal.
Segecic, who has scored six goals and notched two assists across all competitions since joining Pompey, had previously said on multiple occasions he wanted to play for the Socceroos at the World Cup.
That has clearly changed.
Segecic follows in the footsteps of Canberra-born and Australian-raised Josip Simunic, who opted to play for Croatia over Australia in 2001.
It is unclear how soon Segecic could line up for his country of choice, given the talent available to world No.11 Croatia, who have friendlies against Colombia and Brazil this month.
The attacker follows midfielder Nectarios Triantis (Greece) in attending a Socceroos camp then switching nationalities. Triantis has since debuted for Greece.
Segecic's decision is a blow to Australia's attacking stocks, but Popovic has on multiple occasions made it clear he won't be "selling" the Socceroos shirt to players eligible to play for multiple countries.
While Alex Robertson has committed to Australia, the Socceroos are still awaiting a decision from Italy youth international Cristian Volpato.
The 22-year-old knocked back Graham Arnold's approaches ahead of the 2022 World Cup.
But Volpato has warmed to the idea of playing for Australia, and met with Popovic and assistant Paul Okon in Italy last month.
"We're not selling the shirt," Popovic said.
"There were no ultimatums or any pressure - it was a discussion, and if he wants to play for Australia, whether that's, let's say, three months, six months, a year … they're young men, you have to respect the decisions they have to make.
"He's indicated he's open to the idea, now the rest is just for him to decide."
